A standing rigging system is used for stabilizing the mast of a sailboat. Standing rigging systems, which encompass continuous and discontinuous standing rigging systems, typically have rigging terminals or terminal fittings to terminate tension members, or rigging, for example, stays and shrouds. Rigging terminals are typically constructed with steel, cobalt, or titanium metals, and are formed with couples to connect parts of the rigging system together and to the body of the boat. Design of rigging terminals and material selection for rigging can significantly affect performance of the sailboat as a whole. The weight, size and shape of rigging terminals may especially affect the performance of discontinuous standing rigging systems. This is due to the fact that discontinuous standing rigging systems require more discrete or discontinuous segments of tension members, and therefore a greater number of rigging terminals.
Material selection and methods of assembling standing rigging such as stays, shrouds, and other tension members, and rigging terminals for use on sailboats typically have the following design goals: minimize weight, minimize size (for ease of installation), maximize elastic modulus, maximize tensile strength, and minimize aerodynamic drag. In choosing an appropriate material for constructing the tension members, there often is a trade off between these goals. For example, standing rigging made of synthetic polymer fibers, such as poly-p-phenylenebenzobisoxazole (PBO) fibers, can provide performance advantages by way of weight reduction over standard stainless steel rigging. Synthetic polymers, however, provide less stretch resistance than stainless steel. As a result, a greater cross-sectional area of synthetic fibers is required to construct standing rigging with an equivalent stretch resistance. Consequently, to date, PBO rigging have been significantly larger in cross-sectional area than equivalent steel rigging. Similarly, rigging terminals for PBO rigging generally are larger than their steel rigging counterparts.
Composite carbon fiber is another class of synthetic fibers that have been used to construct rigging for standing rigging systems. Composite carbon fibers typically are formed into pultruded carbon rods of about 1 mm in diameter. These carbon rods are then assembled into rigging terminals using a precisely controlled tension apparatus that attempts to equally distribute the tension load among the carbon rods. Generally, the higher the tensile strength requirement to resist a stay load, the higher the number of pultruded carbon fibers are required. Therefore, a predetermined number of pultruded carbon fibers are bundled into a given rigging terminal based on its tensile strength requirement. However, in rigging constructed with pultruded rods, unwanted spaces can form between the rods, making the total cross sectional area larger than a comparable bundle of PBO fibers. According to U.S. Pat. Nos. 6,886,484 and 6,848,381, spaces between pultruded rods can be reduced by forming hexagonal rods and assembling the hexagonal rods into rigging terminals. Consequently, rigging and rigging terminals constructed with pultruded carbon rods are also larger in cross-sectional area than their steel rigging counter parts.
While use of the above-described synthetic materials can reduce total weight of standing rigging systems by up to about 75% over the industry standard Nitronic stainless steel rigging, current design and construction techniques for composite rigging require the use of additional material having a larger cross-sectional area, due to the lower elastic modulus of composite materials as compared to stainless steel. This can result in a larger aerodynamic profile (drag) for the composite rigging. Also, when this composite rigging is assembled inside a mast, a larger internal mast space may be required and larger holes on the body of the mast may be required to accommodate the larger cross-sectional areas of the composite rigging and attendant large rigging terminals. This weakens the mast enclosure and increases the aerodynamic profile of the mast.
FIGS. 1A-1D (PRIOR ART) depict a prior art method of constructing rigging of a predetermined length and assembling terminals using composite fibers such as PBO fibers. First, as shown in FIG. 1A, a plurality of loops of PBO fibers 10 are wound around two spools 11A and 11B to form a sling 12 of PBO fibers. The spools 11A and 11B are held at a predetermined distance from each other to ensure that the loops of fibers forming the sling are held at an approximately equal tension. Referring to FIG. 1B, one end of the sling 12 is then formed into an end fitting with an “eye” opening 13. A temporary “eye” mold 17 may be used to hold the fibers of the sling 12 in place. Once formed, this “eye” opening provides a means to connect the rigging terminal to other parts of the standing rigging system, for example, the mast, or the body of the boat. Due to the nature of PBO fibers, a large-diameter “eye” mold is required to prevent the PBO fibers from breaking around the spool. A spacer 14 may be inserted adjacent to the “eye,” and the PBO fibers are combined to form a rigging tension member. To protect the newly formed end fitting from environmental damage such as saltwater ingress and UV radiation, a molded plastic cover 15 may be slipped over the PBO fibers, as shown in FIG. 1C. Additionally, as shown in FIG. 1D, a braided anti-UV cover 16 may be applied to cover the PBO fibers where they are initially joined together after the wedge.
According to the above design depicted in FIGS. 1A-1D, PBO fibers with a larger cross-sectional area, as compared to equivalent steel rigging, are required to construct a rigging with a stretch resistance equivalent to steel rigging. Consequently, rigging terminals with the “eye” opening are undesirably larger than rigging terminals for steel rigging.
FIGS. 2A-2C (PRIOR ART) depict a prior art method of constructing rigging and rigging terminals using pultruded carbon fiber rods. Referring to FIG. 2A, a bundle of pultruded carbon fiber rods 21 is fed through a terminal fitting 22 with a conical interior space. A conical insert 23 is placed inside the terminal fitting to compress the carbon fiber rods toward the tapered end of the terminal fitting 22, as indicated by an arrow 24a. As shown in FIG. 2B, the conical insert 23 has been completely inserted into the interior space of the terminal fitting 22 while applying a compressive force against the carbon fiber rods 21 against the interior wall 25 of the terminal fitting 22. The compressive force is designed to create enough friction between the carbon fiber rods 21 and the conical insert 23 and between the carbon fiber rods 21 and the interior wall of the terminal fitting 22 such that the tension load on the carbon fiber rods 21 does not pull the rods 21 out of the terminal fitting 22. Additionally, referring to FIG. 2B, a terminal end 26 includes a threaded portion 28 to be screwed into the terminal fitting 22 to provide further compression of the carbon fiber rods 21 to secure the rods inside the terminal fitting 22. An “eye” opening 27 can be formed on the terminal end 26 for connecting with other rigging elements. An example of this arrangement is provided in U.S. Patent Application Publication US 2007/0295256 to Sjostedt et al., in which FIGS. 15 and 16 depict a termination end in which composite rods are retained in a conical plug, and the plug is placed inside a housing portion with a threaded end. Alternatively, as depicted in FIG. 2C, an adhesive, such as epoxy, may be applied to the surface of the conical insert 23 to provide additional holding strength through glue shear.
However, the above design depicted in FIGS. 2A-2C results in rigging and rigging terminals of relatively large cross-sectional areas as compared to equivalent steel rigging and attendant rigging terminals. A further problem with pultruded carbon fiber rods occurs during assembly, where existing processes generally do not provide a consistent load distribution without the use of complex and costly apparatuses for equally distributing the tension load among the rods.
It would be desirable to provide rigging and rigging terminals made of composite materials such as carbon fibers in which the rigging and terminals have cross-sectional areas equivalent to steel rigging, but with a significantly reduced weight, and where the rigging and terminals have a substantially consistent load distribution, without forming unwanted spaces between the fibers.